1. Field of the Invention
This invention relates to a thyristor ac to dc power converter.
2. Description of the Prior Art
Thyristor power converters have been used in recent years for implementing speed control in dc electric motors. One form of such a converter is a static Ward Leonard speed control system as shown in FIG. 1 of the accompanying drawings, wherein reference numeral 1 designates a three-phase transformer serving as a 50/60 Hz power supply. R, S, and T are the Wye connected secondary windings of the transformer, 2 designates internal power supply reactances each having a value of L.sub.1, 3 designates ac control system reactors each having a value of L.sub.2, and 4 is a power conversion unit comprising thyristors 4a-4f. Reference numeral 6 designates a dc motor armature having a series input impedance 5, and 7 is a field coil. Two sets of forward and reverse oriented thyristor units 4 are normally used; only one set is shown in FIG. 1 for clarity. Since this circuit arrangement is well known in the art, no description of its operation will be given.
FIG. 2 shows the waveform of a phase voltage at imaginary line A--A at the input of the thyristor unit 4. Reference character T denotes an interval of time (in general about 0.1 m sec) which corresponds to the commutation overlap angle. The depth of the undesired voltage notch produced during each commutation or thyristor triggering sequence is a function of the values of the reactances L.sub.1 and L.sub.2, and can be expressed by the following equation: ##EQU1## where E.sub.0 =the maximum voltage, and
E.sub.1 =the notch voltage.
When power is supplied from an emergency generator having a relatively high reactance value L.sub.1, as during a power failure, the voltage E.sub.1 becomes small and hence the voltage notch is considerably deepened. The IEEE Guide for Harmonic Control and Reactive Compensation of Static Power Converters, January 1979, discusses notch voltage characteristics in detail, and this paper is incorporated herein by reference.
If a component such as a unijunction transistor is connected to the circuit at line A--A, the voltage notches are liable to cause malfunctioning since unijunction transistors are very susceptible to abrupt voltage drops.
From the foregoing equation it may be easily seen that the depth of the voltage notches may be reduced by increasing the value of the reactance L.sub.2. The larger the reactance L.sub.2, however, the greater the size of the ac reactors 3. The results in an increased commutation overlap angle and an associated increase in the time interval T, which leads to commutation failures in the thyristors 4a-4f.
One prior art approach to solving the notch problem is shown in FIG. 3, wherein a grounded capacitor 8 is connected at the input of the power conversion unit 4. While the capacitor 8 acts to eliminate the voltage notches, it also gives rise to resonances by coaction with the reactances L.sub.1, L.sub.2, causing the voltage waveform to become rippled as illustrated in FIG. 4. Such ripples adversely affect other circuit elements and components connected at line A--A. The capacitor 8 is also easily overloaded by harmonic current components fed back from the thyristor conversion unit 4. To prevent such overloading or to remove the ripples from the voltage waveform, a reactor may be connected in series with the capacitor 8. With such an expedient, however, the voltage notches are again increased.